mesa: create/use a fallback texture when bound texture is incomplete

When a GLSL sampler reads from an incomplete texture it should
return (0,0,0,1).  Instead of jumping through hoops in all the drivers
to make this happen, just create/install a fallback texture with those
texel values.

Fixes piglit/fp-incomplete-tex on i965 and more importantly, fixes some
GPU lockups when trying to sample from missing surfaces.  If a binding
table entry is NULL, it seems that sampling sometimes works, but not
always (lockup).

Todo: create a fallback texture for each type of texture target?

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